Stylus pointing devices enable information to be input to a host electronic device. When the tip of the stylus is placed in close proximity to a drawing and/or display surface of the host device, the position of the tip may be determined by the host by a variety of methods, including the influence of the stylus on the electrical properties of the tablet (i.e., via electromagnetic induction, changes in electrical resistance, electrical capacitance, and the like); the optical properties of the tablet; or by ultrasonic positioning.
Some stylus pointing devices also provide a force, or pressure, output indicative of how hard a user is pressing the stylus against the drawing/display surface of the host electronic device. A variety of force sensors have been used. In many, the stress applied to the tip of the stylus is applied to an elastic element, resulting in a movement or strain of the element that, in turn, produces a change in an electrical property of a sensing circuit. In one approach, two plates of a capacitive sensor are held apart by a mechanical spring. The stress applied to the tip of the stylus compresses the spring and produces movement of one plate. However, this approach requires the use of a mechanical spring to bias the two plates of the capacitive sensor apart. However, mechanical springs are subject to fatigue, and spring rates may vary with temperature, age, etc.
Accordingly, it would be useful, therefore, to provide a capacitive sensor that does not require the use of a mechanical spring and is suitable for use in a stylus pointing device.